1. Field of the Invention
The present invention generally relates to a semiconductor device, and more particularly to a stacked semiconductor device having a polarization enhanced tunnel junction formed on a non-polar semiconductor unit and a further semiconductor unit formed above the polarization enhanced tunnel junction.
2. Description of Related Art
One of the methods for increasing emission efficiency of a light-emitting diode (LED) is using tunnel junctions to stack up two or more LEDs. The stacked LEDs emit more light than a single LED, and thus exhibit increased brightness. The tunnel junction may also enhance current spreading such that more carriers are available in an active layer for recombination. Furthermore, stacked LEDs have fewer electrode contacts than individual LEDs in total, therefore saving more area and decreasing electromigration.
A conventional blue-green LED is primarily made of gallium nitride (GaN), which has a hexagonal structure, and the LED thus possesses polarization. The gallium nitride in commercialized blue-green LEDs is commonly grown on a C-plane sapphire substrate. Atomic charges along the flowing direction of carriers according to the aforementioned structure and growth, however, are asymmetrical, and hence generate a built-in electric field. The built-in electric field is mainly caused by the following two factors: (1) the charge of group III elements is different from the charge of group V elements; (2) the piezoelectric field caused by lattice mismatch between GaN material and the substrate (commonly known as quantum confine Stark effect (QCSE)). The QCSE causes quantum well bandgap skew, and thus reduces the probability of overlap between electron and hole wave functions, therefore reducing emission efficiency.
A need has thus arisen to propose a novel semiconductor device with an improved tunnel junction and without a built-in electric field to increase efficiency of the semiconductor device.